Although it will become evident to those skilled in the art that the leads described are applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the leads and their background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers or defibrillators for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac stimulator and the heart tissue which is to be stimulated. As is well known, the leads connecting such cardiac stimulators with the heart may be used for pacing, for sensing electrical signals produced by the heart, for defibrillation, or for a combination of those procedures in which case a single lead serves as a bidirectional pulse transmission link between the stimulator and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the myocardium. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, maybe coiled, conductor or plurality of conductors surrounded by an insulating tube or sheath typically couples the connector pin at the proximal end and the electrodes at or near the distal end. An epicardial lead is similar to the endocardial lead in terms of construction; however, the lead is connected to the myocardium via the outer surface of the heart.
Implantable cardiac stimulation leads may take the form, for example, of pacemakers capable of pacing and sensing in at least one chamber of the heart. Indeed, embodiments may relate to a programmable dual chamber pacemaker wherein the basic configuration of the pacemaker, e.g. unipolar or bipolar, can be changed, including the grounding configuration and ground potentials used within the pacemaker.
Generally, a heart stimulator uses one or two flexible leads having one end connected to the device and the other end connected to electrodes placed in close proximity to the heart. These leads are used to stimulate or pace the heart. Also, these leads are used to sense the heart activity by picking up electrical signals from the heart.
In order to properly pace or sense or defibrillate, the cardiac stimulator device has to be able to deliver a stimulating pulse to the heart or to sense an electrical signal from the heart. This requires that there be an electrical return path. If, within a given heart chamber, a unipolar lead is used containing a single conductor, the return path comprises the conductive body tissue and fluids. The return path is connected to the stimulator device by connecting the stimulator device's electrical common or ground to the stimulator's metal enclosure, typically referred to as the case or housing. The case or housing, in turn, makes contact with the body tissue and/or fluids. Pacing or sensing using the pacer case or enclosure as part of the electrical return path is known as unipolar pacing or sensing.
An alternative solution to using a unipolar lead in a given heart chamber is to use a double lead/electrode in the heart chamber, known as a bipolar lead. In a bipolar lead, a second conductor is spiraled over and insulated from a first conductor along the length of the lead. At the distal end of the lead, one of the conductors is connected to a first electrode, referred to as the “tip” electrode, and the second conductor is connected to a second electrode, referred to as a “ring” electrode. The ring electrode is generally situated about 10 to 20 mm proximally from the tip electrode. The tip electrode is typically placed in contact with heart tissue, while the ring electrode is in electrical contact with the blood, and in some instances can also be in contact with heart tissue. Because body and heart tissue and fluids are conductive, the ring electrode of a bipolar lead, in contact with the body fluids or tissue, serves as an electrical return for both pacing and sensing. Pacing or sensing using the lead ring electrode and associated lead conductor as the electrical return path is known as bipolar pacing or sensing.
There are numerous factors to consider when deciding whether unipolar or bipolar pacing and/or sensing should be used. Bipolar pacing has, in general, the advantage of requiring slightly less energy than unipolar pacing, and with regard to sensing, bipolar pacing impedance is usually greater than unipolar impedance. Further, bipolar sensing is less prone to far-field signals, crosstalk, and myopotential sensing than is unipolar sensing since its dipole is so much smaller. Crosstalk generally refers to a pacer mistakenly sensing a heart activity in one heart chamber immediately after the other chamber is paced. Bipolar sensing reduces crosstalk resulting from a pacing stimulus in the opposite chamber. Bipolar pacing is preferred if pectoral or diaphragmatic stimulation occurs.
Radio frequency energy can be a potentially dangerous phenomenon for a patient having an implantable medical device. For example, during Magnetic Resonance Imaging (MRI), localized heating effects have been observed near metallic implants in the body. The intra cardiac lead body in an implantable cardiac device system has metallic conducing wires that transmit electrical pulses to the desired location in the heart. During MRI, localized heating has been observed due to the presence of these leads in the body, particularly at the tip of the intra cardiac lead. In many cases, heating is extreme and potentially dangerous.
Radio frequency energy that is inductively coupled into the lead may cause intense heating along the length of the wire and at the electrodes that are attached to the heart wall. This heating may be sufficient to ablate the interior surface of the blood vessel through which the wire lead is placed, and may be sufficient to cause scarring at the point where the electrodes contact the heart, or elevate the patient's body temperature and cause fever.
Thus, there is a need to provide an implantable medical device for which possible damage to the patient or fever due to localized radio frequency heating is minimized or prevented.